Field of the Invention
The present invention pertains to fuel cells, particularly to solid polymer electrolyte fuel cells, and to components and constructions for improving both performance and durability.
Description of the Related Art
Sustained research and development effort continues to be devoted to fuel cells because of the energy efficiency and environmental benefits they can potentially provide. Solid polymer electrolyte fuel cells are particularly suitable for consideration as power supplies in traction applications, e.g. automotive. However, improvements in catalyst technology for cost reduction purposes and in durability after repeated exposure to startup and shutdown remain challenges for automotive applications in particular.
The catalysts in such fuel cells are used to enhance the rate of the electrochemical reactions which occur at the cell electrodes. Catalysts based on noble metals such as platinum are typically required in order to achieve acceptable reaction rates, particularly at the cathode side of the cell. To achieve the greatest catalytic activity per unit weight, the noble metal is generally disposed on a corrosion resistant support with an extremely high surface area, e.g. high surface area carbon particles. However, noble metal catalyst materials are relatively quite expensive. In order to make fuel cells economically viable for automotive and other applications, there is a need to reduce the amount of noble metal (the loading) used in such cells, while still maintaining similar power densities and efficiencies. This can be quite challenging.
One approach considered in the art is the use of certain noble metal alloys which have demonstrated enhanced activity over the noble metals per se. For instance, alloys of Pt with base metals such as Co have demonstrated circa two-fold activity increases for the oxygen reduction reaction taking place at the cathode in the kinetic operating region (amounting to about a 20-40 mV gain). However, despite this kinetic advantage, such catalyst compositions can suffer from relatively poor performance in the mass transport operating regime (i.e. at high power or high current densities). Some of the advantages and disadvantages of such alloys as cathode catalysts are discussed for instance in “Effect of Particle Size of Platinum and Platinum-Cobalt Catalysts on Stability”; K. Matsutani et al., Platinum Metals Rev., 54 (4) 223-232 and “Activity benchmarks and requirements for Pt, Pt-alloy, and non-Pt oxygen reduction catalysts for PEMFCs”, H. Gasteiger et al., Applied Catalysis B: Environmental 56 (2005) 9-35.
Unacceptably high degradation rates in performance can also be an issue in solid polymer electrolyte fuel cells subjected to repeated startup and shutdown cycles. The degradation can be further exacerbated when using low catalyst loadings in the electrodes for cost saving purposes. Often, there is a trade-off between durability and performance in the fuel cell. During the startup and shut-down of fuel cell systems, corrosion enhancing events can occur. In particular, air can be present at the anode at such times (either deliberately or as a result of leakage) and the transition between air and fuel in the anode is known to cause temporary high potentials at the cathode, thereby resulting in carbon corrosion and platinum catalyst dissolution. Such temporary high cathode potentials can lead to significant performance degradation over time. It has been observed that the lower the catalyst loading, the faster the performance degradation. The industry needs to either find more stable and robust catalyst and cathode materials or find other means to address the performance degradation.
A number of approaches for solving the degradation problem arising during startup and shutdown have been suggested in the art. For example, the problem has been addressed by employing higher catalyst loadings, valves around the stack to prevent air ingress into the anode during storage, and using carefully engineered shutdown strategies. Some suggested systems incorporate an inert nitrogen purge and nitrogen/oxygen purges to avoid damaging gas combinations being present during these transitions. See for example U.S. Pat. No. 5,013,617 and U.S. Pat. No. 5,045,414.
Some other concepts involve fuel cell stack startup strategies involving fast flows to minimize potential spikes. For example, U.S. Pat. No. 6,858,336 and U.S. Pat. No. 6,887,599 disclose disconnecting a fuel cell system from its primary load and rapidly purging the anode with air on shutdown and with hydrogen gas on startup respectively in order to reduce the degradation that can otherwise occur. While this can eliminate the need to purge with an inert gas, the methods disclosed still involve additional steps in shutdown and startup that could potentially cause complications. Shutdown and startup can thus require additional time and extra hardware is needed in order to conduct these procedures.
Recently, in PCT patent application serial number WO2011/076396 by the same applicant which is hereby incorporated by reference in its entirety, it was disclosed that the degradation of a solid polymer fuel cell during startup and shutdown can be reduced by incorporating a suitable selectively conducting component in electrical series with the anode components in the fuel cell. The component is characterized by a low electrical resistance in the presence of hydrogen or fuel and a high resistance in the presence of air (e.g. more than 100 times lower in the presence of hydrogen than in the presence of air).
Some catalyst materials are especially vulnerable to such degradation, perhaps due to the catalyst composition employed and/or the nature of the carbon support employed. For instance, PtCo alloy cathode catalyst and/or carbon supported PtCo cathode catalyst may be particularly vulnerable. While a substantial improvement in durability may be expected by incorporating a selectively conducting anode component in such fuel cells, it may nonetheless be expected to be inadequate for practical applications if the cathode catalyst is too vulnerable to such degradation.
For such reasons, alloy catalyst compositions, such as PtCo, are presently considered predominantly for stationary applications and are less attractive for automotive applications which require higher power density.